At the end of its life, any satellite from a constellation of satellites must be deorbited. The attitude and orbit control system of the satellites of the constellation nominally contains a mode used for deorbiting called Orbit Control Mode (OCM).
If, following a major breakdown, the satellite is sent into Safe Hold Mode (SHM) and can no longer attain the deorbiting mode OCM, the satellite can then no longer be deorbited. In fact, the safe hold mode SHM consists of an attitude control mode in which the satellite is kept pointed towards the sun with a slow angular velocity of rotation around the sun-satellite direction by using:                Magneto Torquer Bars (MTB) as main actuator;        reaction wheels to provide gyroscopic rigidity in order to maintain sun-pointing performance in an eclipse of the sun by the Earth despite the cessation of control by the MTBs (since location of the sun is impossible in an eclipse);        Cosine Sun Sensors (CSS) which make it possible to ascertain the direction of the solar vector and therefore the guidance of the satellite; and        Magnetometer sensors (MAG) which make it possible to measure the magnetic field for the estimation of the angular velocity of the satellite and the control of the MTBs.        
The deorbiting mode OCM requires a satellite attitude pointed to the centre of the Earth, which is not in agreement with the guidance of the safe hold mode SHM. In fact, the direction of thrust of the thrusters (THR), the actuators used for generating a force to accelerate or brake the satellite, is in constant rotation, which prevents it from maintaining this axis in the orbital plane and a fortiori from maintaining it collinear with the satellite velocity vector.
The problem to solve is that of controlling the satellite in order to set up adequate conditions for aligning the axis of the THR thrusters with the satellite velocity vector. Such control presupposes measurement of the attitude of the satellite.
Known methods for providing satellite attitude measurements use equipment such as Star Trackers (STR), Gyrometers (GYR) or Infra Red Earth Sensors (IRES). However, these devices are not available on a constellation of satellites in safe hold mode.
In fact, the constellation of satellites in flight does not possess such equipment in safe hold mode. Only basic but robust sensors such as the sun sensors CSS and the magnetometers MAG are available. In safe hold mode, only sun-pointed attitude control is available.
The existing solutions are therefore not compatible with the existing hardware configurations of these satellites. Furthermore, they do not present a sufficient level of robustness to allow use of a greatly reduced set of sensors and actuators.
Thus, a solution for deorbiting a satellite is needed that is compatible with the devices available in safe hold mode.